Who owns IP rights to a video created for employer use but on my own time

It's pretty clear that this IP work was not part of your normal job duties, nor done doing work hours (except for 5% of it) or using the employer's work materials. Unless you signed an employment contract or a contract assigning any IP you created during your employment to them, I think you own this video and its copyright, and that your company needs to compensate you for it. Even if your company claimed that it qualified as and they owned it as a commissioned "work for hire," copyright law requires sales or exclusive licenses to be written, and presumes that the creator is the author and owner of the work unless it's assigned it in writing. So unless and until they get that from you, they can't claim that they own it.

Did any of your casual discussions include what they would pay you? The problem with enforcing an oral contract is that without the material terms - the who, what, where, when, how much - being agreed on, a court can't enforce it.

I suggest coming up with some fair price based on what you've charged others for similar work in the past, less 5% for the amount of work done during company time. If you want it, be sure to include proper credit for yourself and any creative name you use on the video's credits.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.